Add a bower.json

Defined a [bower](http://bower.io/) package.

Currently installing Raphael with Bower pulls down the entire repo. With this `bower.json` you just get:

- `raphael.js`
- `raphael-min.js`
- `license.txt`

Will need to add a version when this gets added to a release. I left it out for now so I can bower install the master zip file.